1. Field of the Invention
This invention relates to a surface plasmon sensor for quantitatively analyzing a substance in a sample by utilizing the occurrence of surface plasmon. This invention particularly relates to a surface plasmon sensor, wherein a light beam coupler means and a metal film, which serves as a sensing means, are spaced apart from each other. This invention also relates to a surface plasmon sensor, which enables measurement with one- or two-dimensional scanning. This invention further relates to a surface plasmon sensor provided with an improved light beam coupler means.
2. Description of the Prior Art
In metals, free electrons vibrate collectively, and a compression wave referred to as a plasma wave is thereby produced. The compression wave occurring on the metal surface and having been quantized is referred to as the surface plasmon.
Various surface plasmon sensors for quantitatively analyzing a substance in a sample by utilizing a phenomenon, in which the surface plasmon is excited by a light wave, have heretofore been proposed. As one of well known surface plasmon sensors, a surface plasmon sensor utilizing a system referred to as the Kretschman arrangement may be mentioned. The surface plasmon sensor utilizing the system referred to as the Kretschman arrangement is described in, for example, Japanese Unexamined Patent Publication No. 6(1994)-167443.
Basically, the surface plasmon sensor utilizing the system referred to as the Kretschman arrangement comprises (i) a prism, (ii) a metal film, which is formed on one surface of the prism and is brought into contact with a sample, (iii) a light source for producing a light beam, (iv) an optical system for causing the light beam to pass through the prism and to impinge upon the interface between the prism and the metal film such that various different angles of incidence may be obtained with respect to the interface, and (v) a photo detecting means capable of detecting the intensity of the light beam, which has been totally reflected from the interface, with respect to each of the various different angles of incidence.
In order for various different angles of incidence to be obtained, a goniometer, with which a light beam irradiating system is rotated, may be employed. The goniometer is disclosed in, for example, Japanese Unexamined Patent Publication No. 6(1994)-50882. Alternatively, an optical system may be employed, with which a light beam having a comparatively large beam diameter is converged on the interface between the prism and the metal film such that the light beam may contain components impinging at various different angles of incidence upon the interface. In the former case, the light beam, which has been reflected at various different angles of reflection from the interface in accordance with the deflection of the incident light beam, may be detected with a small photodetector, which moves in synchronization with the deflection of the light beam, or may be detected with an area sensor extending in the direction, along which the angle of reflection of the light: beam changes. In the latter case, the light beam may be detected with an area sensor extending in a direction such that the area sensor can receive all of the light beam components having been reflected at various different angles of reflection from the interface.
When a light beam impinges at an angle of incidence .theta., which is not smaller than the total reflection angle, upon the metal film, an oozing wave, which is referred to as an evanescent wave, occurs in the metal film serving as the reflection surface. The evane scent wave has an electric field distribution in the sample, which is in contact with the metal film, and the surface plasmon occurs at the interface between the metal film and the sample. In cases where the wave vector of the evanescent wave, which has occurred when the light beam composed of a P-polarized light component impinges upon the metal film, coincides with the wave vector of the surface plasmon and wave number matching is obtained, the evanescent wave and the surface plasmon resonate, and energy of the light transfers to the surface plasmon. As a result, the surface plasmon is excited. At this time, the intensity of the light, which is totally reflected from the metal film, becomes markedly low due to the transfer of light energy.
Therefore, with the surface plasmon sensor described above, the light beam is caused to impinge upon the metal film at various different angles of incidence .theta.. Also, with respect to each of the various different angles of incidence .theta., the intensity of the light beam, which has been totally reflected from the metal film, is detected. In this manner, an angle of incidence .theta..sub.sp (hereinbelow referred to as the attenuated total reflection angle or the ATR angle) at which the phenomenon of marked decrease in intensity of reflected light beam occurs, can be found. A resonance wave number K.sub.sp can then be derived from ATR angle .theta..sub.sp and the wave vector K.sub.1 of the incident light in accordance with the relationship, K.sub.sp =K.sub.1 sin .theta..sub.sp. If the wave number K.sub.sp of the surface plasmon is found, a dielectric constant of the sample can be calculated. Specifically, the formula shown below obtains. ##EQU1## wherein .omega. represents the angular frequency of the surface plasmon, c represents the light velocity in a vacuum, .epsilon..sub.m represents the dielectric constant of the metal, and .epsilon..sub.s represents the dielectric constant of the sample.
If the dielectric constant .epsilon..sub.s of the sample is found, the concentration of a specific substance contained in the sample can be calculated from a predetermined calibration curve, or the like. Accordingly, the specific substance contained in the sample can be quantitatively analyzed by finding the ATR angle .theta..sub.sp, at which the intensity of the reflected light beam becomes low.
In the example of the surface plasmon sensor described above, the metal film, which serves as the sensing means, is formed directly on one surface of the prism, which serves as the light beam coupler. However, actually, for reasons of apparatus constitution, the surface plasmon sensor has the constitution such that the metal film may be formed as a film independent of the coupler. Specifically, the metal film is formed on one surface of a transparent substrate, which may be constituted of glass. (The combination of the transparent substrate and the metal film formed on the transparent substrate will hereinbelow be referred to as the sensor unit.) Also, the other surface of the transparent substrate is brought into close contact with the coupler. Heretofore, in cases where such a separation type of constitution is employed, a refractive index matching liquid is coated on the joint area between the sensor unit and the coupler, and the sensor unit and the coupler are thereby brought into close contact with each other with the refractive index matching liquid intervening therebetween, such that the adverse effects of reflection and multiple reflection due to an air layer at the joint area may be eliminated.
However, with the constitution in which the refractive index matching liquid is coated on the joint area between the sensor unit and the coupler, and the sensor unit and the coupler are thereby brought into close contact with each other with the refractive index matching liquid intervening therebetween, the problems occur in that considerable time and labor are required to mount and dismount the sensor unit. Since the exchanging of the sensor unit is carried out very frequently, it is desired that the exchanging of the sensor unit can be carried out easily. Also, the thickness of the layer of the refractive index matching liquid between the coupler and the sensor unit cannot easily be kept to be equal to a predetermined value. Nonuniformity in thickness of the layer of the refractive index matching liquid, which nonuniformity occurs at the time of exchanging of the sensor unit, or the like, adversely affects the detection accuracy of the surface plasmon sensor.